There are just days left to file a self-assessment tax return to HMRC and avoid a £100 fine – and to make matters even more urgent, the tax office’s phone lines will be closed on the final day of the deadline.

Every January, HMRC sets its self-assessment deadline for those who need to submit a return to cover additional taxes they may owe. This is especially important for people who have received extra income on the side, rent a property, sold assets, are self-employed or who received Child Benefit and earn more than the threshold for repayment.

Those who fail to submit and pay their tax return by midnight on Saturday, January 31, 2026 face an automatic £100 fine, with extra interest charges on top for as long as the money remains unpaid. If you leave it for months without paying, you could rack up hundreds or even thousands in fines and unpaid tax interest charges. That’s because you will be charged 5% of the total tax owed after 30 days, six months and 12 months, which could be thousands.

HMRC has helplines open to assist people in filing their self-assessment tax returns, but these are only open on weekdays, so if you run into trouble filing, you won’t be able to get help if you leave it to the last minute.

HMRC said about its helpline: “Our phone helpline is open Monday to Friday in January. It will close at 6pm on Friday 30 January and reopen at 8am on Monday 2 February.”

There is an AI assistant and webchat support, but this will also close before the deadline.

It added: “Our digital assistant is available 24 hours a day, 7 days a week.

“If it cannot help, webchat support will be available from 8am to 4pm on Saturday 31 January.”

To call HMRC before the deadline for help with filing, the number is 0300 200 3310.

According to HMRC, more than 3.3million people are yet to file their self-assessment with a week to go. 

It is urging taxpayers and agents who are yet to file to act now or risk missing the January 31 deadline and face an automatic £100 penalty.

Myrtle Lloyd, HMRC’s chief customer officer, said: “Don’t leave it until deadline day. Filing now will give you peace of mind that your tax return is completed and if you have tax to pay, you have a week to arrange payment.

“If you’re worried about paying your tax bill, you may be able to set up a payment plan online – search ‘difficulties paying HMRC’ on gov.uk.”